1. Field of the Invention
The present invention relates to an element for detecting the amount of lapping that is used in a lapping step of a stacked structure of layers, a wafer that has such an element, and a method for lapping a stacked structure of layers using such an element. The present invention particularly relates to the structure of an element for detecting the amount of lapping that is used in a lapping step of a thin film magnetic head that is used in a hard disk drive.
2. Description of the Related Art
A thin film magnetic head that is used in a hard disk drive is manufactured through various manufacturing processes, which are generally classified into a wafer process, a slider process, and an assembly process. The wafer process involves forming a read head portion (magnetic field detecting sensor) and a write head portion on a wafer by means of thin film manufacturing technology. The slider process involves dicing the wafer into bars, lapping the bar in order to form an air bearing surface, and separating the bar into individual sliders. The assembly process involves incorporating the finished slider into an end product, such as a head gimbal assembly. In this specification, a surface of a bar or of a slider, which has not yet been lapped and which is to be formed into an air bearing surface after lapping, is called a lapping surface.
In the slider process, many sliders having predetermined dimensions are produced from a single wafer through a dicing process and a lapping process. Among various steps of the slider process, the lapping process for lapping the lapping surface and for forming the air bearing surface is important because the process controls the characteristics of the slider, particularly the characteristics of the read head portion. Consequently, accuracy in the order of nanometers is required in the lapping process, and various methods have been proposed. According to prior art, a method is known that involves providing elements for detecting the amount of lapping in spacing portions rather than in slider portions. The slider portions refer to portions which are actually to be formed into sliders, and the spacing portions refers to portions which are arranged between the slider portions and along which a bar is diced into sliders in a later step. The method further includes lapping the lapping surface while measuring the resistance value of the detecting elements in order to achieve desired dimensions of the read element. See, for example, Japanese Patent Laid-Open Publication No. 2002-245606. The outline of this lapping method will be briefly described.
In the wafer process, the spacing portions are normally provided on the peripheries of the slider portions. The spacing portions extend in two directions on the wafer. When the wafer is diced into bars along the spacing portions in one direction, the slider portions and the spacing portions that cross the spacing portions along which the wafer is diced are alternately arranged in each bar. Resistive films, which are called RLG (Resistance Lapping Guide) or ELG (Electric Lapping Guide) and which are used when the lapping surface is lapped, are formed in advance on the spacing portions which are arranged adjacent to the slider portions when the wafer is diced into bars. The resistive films are formed such that the top surfaces thereof are located beyond the position of the bar that is to be the air bearing surface, just as in the case of the read head portions. Both ends of the resistive film are electrically connected to two pads that are provided on the spacing portions of the wafer. When the lapping surface is lapped, the read head portions in the slider portions and the resistive films are simultaneously lapped, and the electric resistance of the resistive films changes as the cross section of the resistive films becomes small. The two pads are connected to a measuring device via wire bonding or probes in order to measure the electric resistance. The amount of lapping of the read head portions can be controlled by establishing the relationship between the resistance value of the resistive film and the amount of lapping in advance and by monitoring the resistance value.
This method has the advantage that damage to the read elements that may be caused by the ESD (Electro Static Discharge) can be prevented because the method utilizes portions that are finally cut and removed, rather than the portions that are to be formed into elements. Also, this method is effective when accurate measurement of the element resistance is difficult due to what is called smearing, which is caused by lapping an element, such as a TMR (Tunnel Magneto Resistive) element. According to Japanese Patent Laid-Open Publication No. 2002-245606, two pads in the form of triangles are arranged in a manner in which the long sides of the triangles are adjacent to each other so as to form a rectangle as a whole. This method reduces the possibility of entanglement or interference with adjacent wires that are connected to the pads, and therefore facilitates the arrangement of pads in a small area.
A pad having a certain size is required in order to ensure that the pad is connected with the measuring device via wire bonding or a probe. When wire bonding is used, a pad having a certain size is required due to restrictions on the wire diameter for wire bonding, accuracy of a wire bonding machine, etc. When a probe is used, pads need to be arranged at a specific interval due to restrictions on the probe diameter. This increases the area for arranging the pads. Consequently, a spacing portion is required to have a certain area in order to provide sufficient area for arranging the pads in prior art. However, an increase in the size of the spacing portions leads to a decrease in the number of sliders that can be produced from a single wafer. Since the size of spacing portions is constant irrespective of the size of sliders, a further reduction in the size of sliders, which is expected in the future, will relatively increase the ratio of the spacing portions in a wafer, which will limit cost reduction that is achieved based on high-density integration and the resultant increase in the number of elements which are produced from a single wafer. Further, a reduction in the size of a slider leads to a reduction in the height of a slider (the dimension of a slider that is measured in a direction perpendicular to the air bearing surface). This makes it difficult to provide two pads without significantly influencing the measurement. In case of a Femto size slider, which has a slider height of 230 μm, which is 70 μm smaller than that of a pico-slider, there is a limit to improvements or modifications to the pad shape or to the pad arrangement.
It may be possible to provide pads in the spacing portions between adjacent bars and to dice a wafer into blocks each having a plurality of bars that are arranged in parallel. In this case, each bar is lapped when it is in the form of a block, and when one row of the bars is lapped, the row is separated by dicing, and a similar step is repeated. It may be easy to reduce the width of the spacing portion within each bar in this method. However, it is difficult to provide pads for the last bar in the block, which may lead to deterioration in lapping accuracy. Furthermore, it may be difficult to reduce the width of the spacing portions between adjacent bars.